In this article, we seek to explain both the origins of gender mainstreaming as a 'policy frame' in International Relations, as well as the variable implementation of mainstrearning over time and across various international organizations. We emphasize that in the years since the UN Fourth World Women's Conference in Beijing (1995), mainstrearning has been endorsed and adopted not only by European organizations and governments, but also by nearly every important international organization, and we compare the adoption and implementation of mainstrearning in two international organizations, the World Bank and the United Nations Development Programme. We suggest, however, that the rhetorical acceptance of mainstreaming by various international organizations obscures considerable diversity in both the timing and the nature of mainstreaming processes within and among organizations. This variation, we argue, can be explained in terms of the categories of political opportunity, mobilizing structures and strategic framing put forward by social movement theorists.